Method of making solid propellant



United States Patent 3,236,703 METHOD OF MAKING SOLID PROPELLANT Wayne A. 'Proell, Chicago, Ill., assignor to Standard Oil 7 Company, Chicago, III., a corporation of Indiana No Drawing. Filed Oct. 7, 1957, Ser. No. 688,779

5 Claims. (Cl. 14919) This invention relates to explosives and in particular materials suitable for use as solid propellants.

The interest in rocketry has brought forth a surge in demand for cheaper materials suitable for use as propellants. Particular interest has developed in the use of ammonium nitrate as the oxidizer forthe solid propellant. Ammonium nitrate is particularly diflicult to work with because of the crystal changes which occur at ordinary atmospheric temperature. In order to obtain a shaped configuration which will not be broken down by the volume change due to crystal structure it is necessary to have a binder which will absorb the volume changes. Owing to the wide range of temperatures on the earths surface and above, solid propellants for military use must retain their configuration in prolonged storage at elevated temperatures as much as on the order of 75 C. Even though the requirements are arduous solid propellants must be cheap in order to compete. Therefore a constant search is being made for binders which will hold the solid salt in the desired configuration and will still permit fairly simple manufacturing techniques.

An object of the invention is an exposive suitable for use as solid propellant. A particular object is a method of making a solid propellant which propellant will meet rigid military specifications. Further, a particular object is an ammonium nitrate base so'lid propellant using a relatively cheap binder. Another particular object is a solid propellant which is made without need for high temperature curing of the solid salt-binder mass to obtain a thermally stable configuration. Other objects will become apparent in the course of the detailed description.

The solid salt which is the oxidizer in the explosivepropellant product of this invention is ammonium nitrate, ammonium chlorate and ammonium perchlorate. The corresponding alkali metal salts may be used for example, sodium nitrate and potassium chlorate. The salts are used in finely divided form in order to expedite mixing of the solid salt and the binder material. The sieve size may be as coarse as mesh. In general it is preferred that the finely divided salt have a sieve size smaller than +100 mesh.

The binder is the reaction product of a hereinafter defined polyester and a hereinafter defined diisocyanate. The polyester used in the binder is the polycondensation product of diglycolic acid and a dihydric alcohol, namely, glycols containing rfirom 2 to 6 carbon atoms such as ethylene glycol, butandiol and hexadiol; polyethylene glycol ethers and polypropylene glycol ethers having a molecular weight of not more than about 200 such as tetraethylene glycol and dipropylene glycol. The polycondensation product is a viscous liquid at ordinary temperatures and has a molecular weight between about 250 and 1000. In addition to the dihydric alcohol the polycondensation reaction may be carried out using some higher alcohol such as, glycerol in order to increase the viscosity of the liquid at a particular molecular weight. A slight excess of the alcohol is used in order to obtain the desired product character. The amount of excess alcohol is dependent upon the particular alcohol used. When using ethylene glycol and diglycolic acid the mol ratio of ethylene glycol to diglycolic acid is between about 1.05 and 1.25, more usually between about 1.05 and 1.10.

3,236,703 Patented Feb. 22, 1966 The polycondensation product is obtained by the conventional means wherein the acid and the alcohol are heated under conditions to continuously remove product water. Since this reaction is very well known it is not deemed necessary to go into detail; however, an excellent summary of polyester production is presented in chapter II of Polyesters and Their Applications, by Bjorksten Research Laboratories, Inc. (Reinhold 1956).

The diisocyanate reactant may be a methylene diisocyanate containing from 4 :to 18 methylene groups, such as tetramethylene diisocyanate, hexamethylene diisocyanate and octodecamethylene diisocyanate. Also the aromatic diisocyanates tolylene diisocyanate and benzidene diisocyanate. Sufficient diisocyanate is used to theoretically react with the unreacted functional groups present in the polycondensation product.

It is to be understood that in addition to the solid salt and the hinder the explosive or propellant may contain any of the other additives normally present in exposives based on ammonium nitrate and chlorates. For example, combustion catalyst may be present to expedite the burning of the propellant. Well known catalysts are the inorganic chromium compounds such as ammonium dichromate, and Prussian blue. Carbon may be added to the material; inhibitors to improve the chemical stability of the material at elevated temperatures for example, various aromatic hydrocarbon amines. Surfactants may be present in order to improve the blending of the solid salt and the binder components prior to curing.

The amount of polycondensation product-diisocyanate reaction product present in the finished propellant will be determined by the specifications of the particular product. In general for military purposes between about 18 and- 35 weight percent of the final reaction product is present based on total propellant. The absence of smoke in the burning of solid propellants is frequently a very desirable feature. With the reaction product binder a smoke free propellant is obtainable using between about 20 and 25 weight percent of the binder in the propellant.

The solid propellant is made by admixing the finely divided solid salt and any other solid components with the polycondensation product. This mixing may be carried out at ordinary ambient temperatures such as 20 C. up to a maximum temperature at which it is suitable to work with the particular solid salt. With ammonium nitrate it is generally desirable to operate at temperatures below about C. It is preferred to operate at the lowest temperature which perm-its ready working of the solid materials into the liquid polycondensation product. The mixing is continued until essentially all of the solid particles are coated with the polycondensation product.

The diisocyanate is added to the polycondensation product-salt blend at a temperature such that reaction between the polycondensation product and the diisocyanate is slow. The diisocyanate is thoroughly intermingled into the mass until a plastic mass of material is obtained which is substantially uniform in quality. In general the diisocyanate is mixed in at a temperature between about 20 C. and 40 C. and preferably at ordinary ambient temperature.

After all of the components have been blended together the plastic mass is withdrawn from the mixer and treated to obtain the particular configuration desired. The plastic mass may be placed into molds which are provided to give shapes such as rods or tubes. Also the mass may be placed into shallolw troughs and at the end of the curing time the material may be broken up into irregular shapes for use as blasting powder, etc. In general the plastic mass will set in 3 to 6 hours at ordinary temperatures on the order of 20 to 25 C. The reaction continues and the entire cure may take 2 to 4 days. After the cure has been completed the propellant will retain its shaped configuration even when exposed to temperatures on the order of 75 C. for as long as 30 days.

Example A glass still provided with a condenser and trap-out was charged with 187 grams of diglycolic acid (1.41 mols), 87 grams of ethylene glycol and 4.3 grams of glycerol were then charged to the still. This provides a mol ratio of 1.05 mols of hydroxyl groups per mol of carboxyl groups. The contents of the still were heated and product water continuously removed through the trap-out; this heating was carried out at about 150 C. When the rate of water evolution slowed down the pressure on the still was lowered to about 5 mm. Hg and the temperature in the still raised gradually up to 185 C. The polycondensation product was a viscous liquid at room temperature, i.e., 25 C.

The polycondensation product and ammonium nitrate reduced in size so that the particles were between about 200 mesh and 325 mesh were placed in a mixer. Prussian blue catalyst, 3 weight percent, was also added. The materials were thoroughly intermingled at room temperature. When the materials were uniform as evidenced by visual inspection tolylene diisocyanate, 3.2 weight percent based on polycondensation product, was added to the mixer. Operating at room temperature the diisocyanate was blended into the mixture until a uniform pasty mass had been obtained.

The pasty mass was transferred to a mold capable of producing a cylindrical configuration such as, 8" long by 3" in diameter with a 1 coaxial opening.

The plastic material had attained an initial set in 3 hours. After approximately 2 days the mass had cured to a thermally stable solid material which retained its configuration after exposure to 77 C. for 30 days.

The propellant containing 20 weight percent of the defined binder burned smoke-free and was suitable for use as a smokeless rocket propellant.

Thus having described the invention, what is claimed is:

1. A method for producing solid propellant which method comprises (a) admixing a finely divided salt selected from the class consisting of ammonium nitrate,

ammonium chlorate, ammonium perchlorate and the corresponding alkali metal salts, with a polycondensation product of diglycolic acid and a dihydric alcohol selected from the class consisting of glycols containing from 2 to 6 carbon atoms, polyethylene glycol ethers and polypropylene glycol ethers, said ethers having a molecular weight of not more than about 200, said product being characterized as a viscous liquid with a molecular weight between about 250 and 1000 and said admixing being carried out at a temperature between about 20 C. and 100 C. for a time sufficient to coat essentially all of said particles with said product, (b) adding a diisocyanate selected from the class consisting of methylene diisocyanates containing from 4 to 18 methylene groups, tolylene diisocyanate and benzidene diisocyanate, in about the theoretical amount needed to react with said product, to the mass produced in step (a) at a temperature between about 20 C. and C. and mixing until a substantial uniform plastic mass is obtained, and (c) curing said plastic mass until a hard body is obtained which is characterized by the ability to retain its configuration at temperatures on the order of C.

2. The propellant of claim 1 wherein said polycondensation product is obtained with ethylene glycol and diglycolic acid in a mol ratio of between about 1.05 and 1.25.

3. The propellant of claim 2 wherein said diisocyanate is tolylene diisocyanate.

4. The propellant of claim 1 wherein the binder produced from the polycondensation product and the diisocyanate forms about 20 and 25 weight percent of said propellant.

5. The propellant of claim 1 wherein said salt is ammonium nitrate.

References Cited by the Examiner UNITED STATES PATENTS 2,155,499 4/1939 Lawson. 2,742,672 4/1956 Thomas. 2,780,996 2/1957 Hirsch et al.

CARL D. QUARFORTH, Primary Examiner.

LEON D. ROSDOL, Examiner. 

1. A METHOD FOR PRODUCING SOLID PROPELLANT WHICH METHOD COMPRISES (A) ADMIXING A FINELY DIVIDED SALT SELECTED FROM THE CLASS CONSISTING OF AMMONIUM NITRATE, AMMONIUM CHLORATE, AMMONIUM PERCHLORATE AND THE CORRESPONDING ALKALI METAL SALTS, WITH A POLYCONDENSATION PRODUCT OF DIGLYCOLIC ACID AND A DIHYDRIC ALCOHOL SELECTED FROM THE CLASS CONSISTING OF GLYCOLS CONTAINING FROM 2 TO 6 CARBON ATOMS, POLYETHYLENE GLYCOL ETHERS AND POLYPROPYLENE GLYCOL ETHERS, SAID ETHERS HAVING A MOLECULAR WEIGHT OF NOT MORE THAN ABOUT 200, SAID PRODUCT BEING CHARACTERIZED AS A VISCOUS LIQUID WITH A MOLECULAR WEIGHT BETWEEN ABOUT 250 AND 1000 AND SAID ADMIXING BEING CARRIED OUT AT A TEMPERATURE BETWEEN ABOUT 20*C. AND 100* C. FOR A TIME SUFFICIENT TO COAT ESSENTIALLY ALL OF SAID PARTICLES WITH SAID PRODUCT, (B) ADDING A DIISOCYANATE SELECTED FROM THE CLASS CONSISTING OF METHYLENE DIISOCYANATES CONTAINING FROM 4 TO 18 METHYLENE GROUPS, TOLYLENE DIISOCYANATE AND BENZIDENE DIISOCYANATE, IN ABOUT THE THEORETICAL AMOUNT NEEDED TO REACT WITH SAID PRODUCT, TO THE MASS PRODUCED IN STEP (A) AT A TEMPERATURE BETWEEN ABOUT 20*C. AND 40*C. AND MIXING UNTIL A SUBSTANTIAL UNIFORM PLASTIC MASS IS OBTAINED, AND (C) CURING SAID PLASTIC MASS UNTIL A HARD BODY IS OBTAINED WHICH IS CHARARTERIZED BY THE ABILITY TO RETAIN ITS CONFIGURATION AT TEMPERATURES ON THE ORDER OF 75*C. 